Cellular radio networks comprise applications which require that a subscriber terminal or some other corresponding radio receiver receives synchronized radio signals from various base stations. Such applications include different methods of locating subscriber terminals. An example of such locating methods is an OTD (Observed Time Difference) method based on time differences detected in the reception of signals. In this method a terminal equipment measures differences in times of arrivals of signals transmitted by base stations. The method requires that the base stations transmit signals at the same moment, in other words synchronously, or otherwise data is required on the differences in synchronization (Real Time Difference, RTD) between the base stations if the base stations are not synchronized. The location is carried out based on this data. This method is described in greater detail in Finnish Patent Application 954,705.
Several systems, such as the GSM system, are not synchronized or they are not synchronized sufficiently accurately so that the signals could be used in the location according to the OTD method. In the GSM system, normal channels are divided both on a time division (TDMA, time division multiple access) and frequency division (FDMA, frequency division multiple access) basis. A radio transmitter thus uses a specific time slot on a predetermined frequency for transmitting a normal physical channel. In the GSM system, the base stations transmit radio bursts of a normal channel asynchronously, which means that the transmissions between the base stations are not coordinated such that each base station would transmit a radio burst simultaneously. Further, the aforementioned synchronization differences between the base stations change over time. Therefore the OTD method cannot be used for location without continuous measurement of the synchronization differences. Measurement of the synchronization differences produces more signalling and causes additional error in the accuracy of the location.
One suggested solution is to synchronize all the radio transmitters with each other by means of a satellite-based locating system (global positioning system, GPS), in which case a GPS receiver would be installed at each base station. This arrangement may cause problems in the GSM system since the system utilizes hierarchical clocks. This means that a base station controller guiding a base station obtains timing from higher network elements and delivers it to the base stations. If a GPS receiver were used for the timing of the base station transmission, the entire timing of the GSM system would be confused.